FERTILISERS (GENERAL METHODS) 121 been completely removed, the soda solution will fill the part of the tube below the clip immediately and completely. Shortly after, when a little water has condensed in the conical beaker so as to cover the extremity of the other capillary tube, the second clip is shut and the name at once removed from beneath the flask. About 15-20 c.c. of ferrous chloride solution are 1 poured into the beaker and allowed to pass into the flask by opening the clip. When the ferrous solution only just covers the tip of the capillary tube, a little hydrochloric acid (D i-i) is poured into the beaker and later a little more, so as to wash out the beaker and displace all the ferrous chloride from the tube, care being taken that no air enters the tube and hence the flask a ; about 10 c.c. of acid is sufficient for this purpose. The clip is closed, the flame replaced under the flask and a graduated tube (100 c.c. reading to 0-5 c.c.) filled with boiled 10% caustic soda solution placed over the end of the gas delivery tube. When the rubber joint of this tube begins to swell, the clip is replaced by the fingers, and as soon as evolution of nitric oxide begins, the joint is left free. The boiling is regulated so that liberation of gas is not too rapid, and when this ceases the graduated tube is closed by the thumb and shaken and then placed in a bath of water. The flask is then rinsed out and the operation repeated in identical man- ner with the solution of the substance to be tested. With sodium (or potas- sium) nitrate or fertiliser containing it, 16-5 (or 20) grams are dissolved in hot water and, if insoluble residue remains, filtered and the residue washed ; the volume is then made up to i litre.2 Of this solution 20 c.c. are introduced into flask a if the nitrate alone is being tested or double or three times, etc., as much in the case of mixed fertilisers, so that the volume of NO collected is about the same as in the control experiment with the pure nitrate. The second tube containing nitric oxide is placed in the same bath of water as the first and when the temperature of the gas is in equilibrium with that of the surrounding air (about i hour is sufficient), the two tubes are immersed in the bath so that the levels inside and outside are the same. The volume of the gas is then read off in each case at the lower edge of the meniscus. When equal quantities of the fertiliser and pure nitrate are used, the percentage (x) of nitrogen is given by the formula K(a x 100) x = where K = the coefficient for reducing the nitrate to nitrogen and has the value 0-1647 f°r NaN03 and 0-1387 for KN03, a — c.c. of NO obtained from the fertiliser. b = c.c. of NO obtained from the pure nitrate. 1 This solution is prepared by placing 200 grams of fine iron filings in a flask with. 100 c.c. of water, and gradually heating the flask on a sand-bath and adding hydro- chloric acid (D i-1) until all the iron is dissolved. The liquid is filtered to get rid of carbon and made up to i litre with boiled water. 2 If the fertiliser contains carbonate, the solution is prepared with water containing hydrochloric acid to eliminate the carbon dioxide. If the fertiliser contains oxalic acid (guano), the latter is rendered insoluble by addition of a little milk of lime.5-80%).